1. Field of the Invention
The present invention relates to a laminate type semiconductor ceramic capacitor with a varistor function, and more particularly, relates to a laminate type semiconductor ceramic capacitor with a varistor function, which uses a SrTiO3 based grain boundary insulated semiconductor ceramic.
2. Description of the Related Art
In recent years, mobile electronic devices such as cellular phones and laptop personal computer, and in-car electronic devices to be installed on cars have become common, and as with the development of electronics technology, a reduction in size with multiple functions has been required for the electronic devices.
In order to achieve the reduction in size with multiple functions for the electronic devices, more semiconductor elements have been used such as various types of ICs and LSI, and consequently, the electronic devices have been losing their noise immunity.
Conventionally, power lines for various types of ICs and LSI are provided with a film capacitor, a laminate type ceramic capacitor, a semiconductor ceramic capacitor, or the like as a bypass capacitor, to thereby ensure the noise immunity for the electronic devices.
In particular, it is commonly the case in car navigation systems, car audio systems, in-car ECUs, etc., that a capacitor with a capacitance on the order of 1 nF is connected to an external terminal, to thereby absorb high-frequency noises.
However, while these capacitors deliver superior performance on the absorption of high-frequency noises, the capacitors themselves do not function to absorb high-voltage pulses or static electricity. For this reason, there is a possibility that if the high-voltage pulses or static electricity are input to the electronic devices, they may cause the electronic device to malfunction or cause the semiconductor elements to be broken. In particular, a lowered capacitance on the order of 1 nF makes the ESD (Electro-Static Discharge) withstanding voltage extremely low (for example, on the order of 2 kV to 4 kV), thereby possibly leading to breakdown of the capacitor itself.
Conventionally, as shown in FIG. 2(a), a zener diode 105 is provided in parallel to a capacitor 104 connected to a power line 103 for connecting an external terminal 101 and an IC 102, or as shown in FIG. 2(b), a varistor 106 is provided in parallel to the capacitor 104, thereby ensuring an ESD withstanding voltage.
However, when the zener diode 105 or the varistor 106 is provided in parallel to the capacitor 104 as described above, the number of components is increased to cause an increase in cost. Moreover, a space for the placement of the components has to be ensured, and there is thus a possibility that an increase in the size of the device may result.
If the capacitor had a varistor function, the need for the zener diode or the varistor will be eliminated, and the ESD withstanding voltage can also be handled by only the capacitor as shown in FIG. 3, thereby making it easier to standardize the design, and thus allowing a value-added capacitor to be provided.
International Publication WO2008/004389 proposes a laminate type semiconductor ceramic capacitor with a varistor function in which a semiconductor ceramic is formed of a SrTiO3 based grain boundary insulated type, the compounding molar ratio m between the Sr site and the Ti site satisfying 1.00<m≦1.020, a donor element is present as a solid solution in crystal grains, an acceptor element is present in a grain boundary layer in the range of 0.5 mol or less (excluding 0 mol) with respect to 100 mol of the Ti element, and the crystal grains have an average grain size of 1.0 μm or less.
According to the International Publication WO2008/004389, the semiconductor ceramic which has the composition described above allows a laminate type grain boundary insulated semiconductor ceramic capacitor with a varistor function to be achieved which provides favorable insulation properties, has a favorable ESD withstanding voltage, and allows the reduction in layer thickness and the reduction in size.